Influence of the compressive stress on the infrared absorption of s p2 -bonded boron nitride in cubic boron nitride thin films

Article abstract of DOI:10.1063/1.2768199

Cubic BN films with a pure cubic phase upper layer were prepared by plasma-enhanced chemical vapor deposition. Infrared spectroscopy was applied to analyze the content of initial s p2 -bonded BN layer in cubic BN thin films under compressive stress. It was found that the peak intensity near 1380 cm-1 attributed to the B-N stretching vibration of s p2 -bonded BN was suppressed by the compressive stress in cubic BN films. The deviation between the measured and calculated peak intensities was found to be linear with the compressive stress when the upper layer of the film is a pure cubic phase layer.

Full text of DOI:10.1063/1.2768199

Influence of the compressive stress on the infrared absorption of s p 2 -bonded boron
nitride in cubic boron nitride thin films
Hangsheng Yang, Yan Zhang, Xiaobin Zhang, and Yabo Xu
Citation: Applied Physics Letters 91, 061907 (2007); doi: 10.1063/1.2768199
View online: http://dx.doi.org/10.1063/1.2768199
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APPLIED PHYSICS LETTERS 91, 061907 ͑2007͒
Influence of the compressive stress on the infrared absorption
of sp2-bonded boron nitride in cubic boron nitride thin films
Hangsheng Yang,a͒ Yan Zhang, and Xiaobin Zhang
State Key Laboratory of Silicon Materials, Department of Materials Science and Engineering,
Zhejiang University, Zheda Road 38, Hangzhou 310027, China
Yabo Xu
Department of Physics, Zhejiang University, Zheda Road 38, Hangzhou 310027, China
͑Received 20 April 2007; accepted 12 July 2007; published online 7 August 2007͒
Cubic BN films with a pure cubic phase upper layer were prepared by plasma-enhanced chemical
vapor deposition. Infrared spectroscopy was applied to analyze the content of initial sp2-bonded BN
layer in cubic BN thin films under compressive stress. It was found that the peak intensity near
1380 cm−1 attributed to the B–N stretching vibration of sp2-bonded BN was suppressed by the
compressive stress in cubic BN films. The deviation between the measured and calculated peak
intensities was found to be linear with the compressive stress when the upper layer of the film is a
pure cubic phase layer. © 2007 American Institute of Physics. ͓DOI: 10.1063/1.2768199͔
Cubic boron nitride ͑c-BN͒ films have significant tech-
nological potential applications because of their excellent
properties, such as high hardness, wide band gap, and
p-/n-type dopability.1,2 Recently, much effort has been made
toward the improvement of c-BN films.3–9 High energetic
ion bombardment is still necessary for c-BN nucleation and
growth, which induced high compressive stress accumulation
up to several gigapascals in film.1 Also, an sp2-hybridized
amorphous BN ͑a-BN͒ layer and a textured turbostratic BN
͑t-BN͒ layer are always deposited on silicon substrate before
c-BN nucleation. The infrared ͑IR͒ spectroscopy is widely
used to evaluate the content of sp2-bonded BN layer in c-BN
films. Quantitative determination of the sp2-bonded BN by
IR was found to be problematic. For example, the absorption
coefficient of the sp2-bonded BN in c-BN films was found to
be much less than that predicted from the nearly single-
crystal properties, and the absorption coefficient of the
sp2-bonded BN peak near 1380 cm−1 changed by a factor of
3 for a modest change in substrate bias during film
preparation.10,11 Recently, we have developed a c-BN film
deposition process by which the upper layer of the c-BN film
could be a pure cubic phase.12,13 Therefore, it is possible to
measure the t-BN content by cross-sectional high-resolution
transmission electron microscopy ͑HRTEM͒.
10% B2H6 diluted with He was introduced gradually from a
water-cooling injection probe into the plasma up to
0.3 SCCM within a 2 min time span. After stabilizing the
plasma at 1 mTorr, the substrate bias was reduced to a sheath
potential of approximately 65–80 V for 3–10 min film
deposition.6,12 Samples were characterized using cross-
sectional HRTEM ͑JEM-4000 FX II and JEM ARM 1250͒
and nonpolarized Fourier transform infrared ͑FTIR͒ ͑JASCO
FT/IR-700͒ with the transmission mode at normal incidence.
The compressive stress accumulated during film growth was
determined from the bending of the silicon substrate using
the Stoney formula.14 The stress induced bending of silicon
substrate was measured by a tenchor profiler 10 ͑KLA-
Tenchor͒.
Figure 1 shows a typical IR spectrum of a c-BN film and
its curve fitting. In many cases, the IR peak position and
intensity determination by the standard method, as the dash
lines shown in Fig. 1, is interfered by the overlap with adja-
cent peaks. In this letter, therefore, the peak intensities and
positions were determined by fitting IR spectra with pure
Lorentzian-type peak functions. With a linear background,
the spectrum is composed of five peaks. The peaks at 781.03
and 1380.78 cm−1 are attributed to the B–N–B bending vi-
bration and B–N stretching vibration of sp2-bonded BN. The
With these as a background, in this letter, we studied the
influence of the compressive stress on the IR absorption of
the initial sp2-bonded BN layer in c-BN films, and our re-
sults revealed that the B–N stretching vibration peak inten-
sity near 1380 cm−1 is suppressed by the compressive stress
accumulated during film growth.
Cubic BN films were deposited on single-crystal silicon
substrates by inductively coupled plasma-enhanced chemical
vapor deposition.5,12 The plasma was generated at 1 mTorr
with an input power of 7 kW. After a series of pretreatments,
the plasma gas composition was changed from pure N2 to
approximately 1.6% N2+98.4% Ar by adjusting the flow
rates of N2 and Ar to 0.3 and 18.0 SCCM ͑SCCM denotes
cubic centimeter per minute at STP͒, respectively. Then,
FIG. 1. ͑Color online͒ FTIR spectrum of a typical c-BN film. The spectrum
is well fitted with five peaks; the dash lines show the traditional peak posi-
tion and intensity determination method.
a͒
Author to whom correspondence should be addressed; electronic mail:
hsyang@zju.edu.cn.
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061907-2
Yang et al.
Appl. Phys. Lett. 91, 061907 ͑2007͒
FIG. 2. ͑Color online͒ HRTEM image of a typical c-BN with a pure cubic
phase upper layer. The interface between the sp2-bonded BN layer and c-BN
upper layer and their layer thicknesses are also shown schematically.
FIG. 4. ͑Color online͒ Deviation ͑Icalc.−Imeasure͒ of the peak intensity is
found to increase with compressive stress; the inset shows the c-BN peak
position increased linearly with the measured compressive stress.
peak around 1076.94 cm−1 is the characteristic transverse op-
tical phonon of c-BN, and this peak position was found to be
located between 1060 and 1100 cm−1 depending on the re-
maining compressive stress, and shifted to high wave num-
ber linearly with increasing compressive stress.15,16 The
weak peak at 1505.17 cm−1 can be attributed to a two-
phonon peak.17 For good curve fitting, an additional peak at
1234.22 cm−1 is necessary; this peak is attributed to y-BN,
which is not yet understood.18
which is considered to be due to the different compressive
stresses in films, since films were deposited under different
substrate biases and for different deposition times.10,19 The
stress conditions inside a c-BN film are complicated due to
the Si/a-BN/t-BN/c-BN multilayer film structure.19,20 For
c-BN film direct deposition on the substrate without a buffer
layer, in situ stress measurement revealed that the total com-
pressive stress increases steadily from the deposition of the
initial t-BN layer to the deposition of the upper c-BN layer,
while within the c-BN layer, the instantaneous compressive
stress remains largely constant.19 In this study, we also de-
posited c-BN films directly on the silicon substrates without
a buffer layer, which suggests that a compressive stress is
accumulated in the t-BN layer.19 For simplification, we stud-
ied the effect of the film apparent stress on the peak intensity
Imeasure near 1380 cm−1 using the c-BN peak wave number as
the indicator of the apparent stress in the film.
Figure 4 shows the relationship between the Icalc.
−Imeasure and the film compressive stress accumulated during
c-BN growth. The Icalc. and Imeasure are distributed randomly
due to the different Tinitial. However, the Icalc.−Imeasure is
found to increase linearly with the compressive stress, which
indicates that the Imeasure is suppressed by the compressive
stress in the film. The inset shows the linear relationship
between the apparent compressive stress and the c-BN peak
position. Since samples were prepared under very similar
conditions with similar nanostructures, it is reasonable to
consider that the c-BN peak shift is induced by compressive
stress only.15,21 Figures 3 and 4 suggest that the Tinitial of a
c-BN film could be deduced from the Imeasure of t-BN near
1380 cm−1 and the c-BN peak position when the upper layer
is a pure cubic phase layer.
Figure 2 shows HRTEM image of a typical c-BN film.
An a-BN/t-BN/c-BN layered structure is clearly observed.
The upper c-BN layer is a pure cubic phase layer; no
sp2-bonded BN was detected on the film surface or on the
grain boundary of individual c-BN nuclei. Note that, in many
cases, c-BN films have a t-BN/c-BN mixture upper layer,
therefore, determination of the sp2-bonded BN content and
its initial layer thickness becomes difficult. Here, the inter-
face between the c-BN and t-BN layers is discernible but not
straight. In this study, the initial sp2-bonded BN layer thick-
ness ͑Tinitial͒ is defined as the area of initial sp2-bonded BN
layer divided by the length of the film/substrate interface in a
cross-sectional HRTEM image. In this way, Tinitial of the film
in Fig. 2 was determined to be approximately 7.8 nm.
Figure 3 shows the Tinitial of six films and their measured
peak intensities ͑Imeasure͒ near 1380 cm−1 correspondingly.
For comparison, a series of calculated peak intensities
͑Icalc.=IlowestTinitial/Tlowest͒ were also shown, where Ilowest and
Tlowest are Imeasure and Tinitial of the sample with the lowest
compressive stress ͑corresponding to c-BN peak position at
1073.75 cm−1 in this study͒, respectively. The calculated in-
tensities are based on the assumption that the intensity is
simply proportional to the layer thickness. No obvious linear
relationship between Tinitial and Imeasure is shown in Fig. 3,
The IR absorbance of a c-BN film is sensitive to and is
often influenced by its composition and nanostructure, for
example, the existence of t-BN in the upper c-BN layer and
the t-BN basal plane ͑0002͒ orientation variation.1,10,13,16,22
In this study, we found that the Imeasure near 1380 cm−1 is also
a function of compressive stress. This could be one of the
reasons that the measured absorption coefficient of t-BN in
c-BN films under compressive stress was much less than that
predicted from the nearly single-crystal properties.10,11 Ac-
cording to the quantum theory of phonon excitation, the IR
absorption coefficient should be proportional to the density
of phonon final states ͑at the center of Brillouin zone͒ which
is related to the phonon dispersion and can be influenced by
FIG. 3. ͑Color online͒ I
near 1380 cm−1 shows no linear relationship
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with its Tinitial. The Icalc. is also shown.
the strain in a material.23 Further works to clarify the mecha-
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061907-3
Yang et al.
Appl. Phys. Lett. 91, 061907 ͑2007͒
Schreck, Nat. Mater. 2, 312 ͑2003͒.
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In conclusion, the effect of the compressive stress on the
IR peak intensity near 1380 cm−1 attributed to the B–N
stretching vibration of sp2-bonded boron nitride in a c-BN
film was investigated. The Imeasure near 1380 cm−1 is revealed
to be suppressed by the compressive stress, at least for c-BN
films deposited by inductively coupled plasma-enhanced
chemical vapor deposition. Our findings also suggest that the
influence of the compressive stress on the IR absorption
should be taken into account when IR was used to quantita-
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stress.
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